Refrigeration systems for preserving perishable materials while those materials are being transported have been used for some time. Different types of refrigeration systems, such as freon-based compressor systems and carbon dioxide systems, have been used over the years.
Carbon dioxide refrigeration systems have been gaining increased use in recent years due to several advantages inherent in such systems. In carbon dioxide refrigeration systems, pressurized liquid carbon dioxide is introduced into pipes which constitute a carbon dioxide distribution system within the refrigeration container. The pipes have a plurality of apertures. When the pressurized liquid carbon dioxide passes through the pipes and apertures, the carbon dioxide pressure drops rapidly, and the carbon dioxide converts to solid carbon dioxide "snow" and carbon dioxide gas.
Typically, the refrigeration container has a compartment that holds the perishable materials to be refrigerated. The distribution pipes are arranged to deposit the "snow" on the ceiling of the lower compartment, as shown in Hill (U.S. Pat. No. 4,704,876). The "snow" refrigerates the lower compartment during the transportation of the perishable materials. As shown in Franklin (U.S. Pat. No. 4,502,293), fans may also be used to increase the circulation of refrigerated air around the lower compartment. Some systems, such as Hill, vent the refrigerated gas into the compartment to increase the refrigerating affect. As the container gradually warms up, the carbon dioxide "snow" sublimates to gas and, together with the gas already present in the container, this gas expands as the temperature in the container rises. Vents to the outside atmosphere are provided to allow the excess carbon dioxide gas to escape from the container.
It can be seen from the above discussion that the carbon dioxide refrigeration systems are very simple mechanically. Unlike freon-based compressor systems, the carbon dioxide refrigeration systems have almost no moving parts to repair or maintain. Also, liquid carbon dioxide is inexpensive, and readily available. With the insulation typically used in a refrigeration container, such as a railcar or a truck trailer, a single carbon dioxide charging of a refrigeration container will allow frozen foods to remain stored for a week or more without operator intervention.
Despite the advantages of a carbon dioxide refrigeration system as represented by the invention disclosed in Hill, there remain a number of problems with such systems. The carbon dioxide distribution system in these systems is inefficient for several reasons. Typical distribution systems only produce about one pound of "snow" for every two pounds of liquid carbon dioxide. Also, when cooling vents to the lower compartment are on the surface to which the snow is applied, there can be a problem with vents to the lower compartment accidentally being plugged by the "snow", thus preventing the refrigerated gas from entering the lower compartment.
Inefficiencies also occur when the excess carbon dioxide gas is vented from the refrigeration container. When the exhaust vent is located low in the container, as it is in Hill, the excess carbon dioxide gas readily escapes the refrigeration container, thus, lowering the pressure within the container. This lower pressure, in turn, increases the rate of sublimation of the "snow" into gas and shortens the length of time that the container will remain refrigerated.
Different problems are represented by perishable materials, such as fresh produce, that cannot be frozen and some of which are intolerant of a high carbon dioxide environment. For such materials, the refrigeration temperature must be limited in the lower compartment, and carbon dioxide gas must be kept out of the lower compartment. These restrictions can prevent the use of refrigeration containers designed only to store and transport frozen materials for the transporting of materials like fresh produce.
As can be seen from the present discussion, it would be desirable to have a carbon dioxide refrigeration system that would allow the use of one refrigeration container for the refrigeration of either frozen materials or materials that cannot be frozen and which may be intolerant of a carbon dioxide environment.
It would also be desirable to have a carbon dioxide refrigeration system that would more efficiently create "snow" in the refrigeration container and distribute refrigerated air. Further, it would be desirable to have a system that would remain sufficiently refrigerated for a longer period of time than is possible with existing systems.
Although the discussion herein relates to carbon dioxide refrigeration systems for transportation containers, it is not intended that the invention be limited to this situation. It will be obvious from the description that follows that the present invention will be useful in other applications with problems common to those described herein.